1. Field of the Invention
The present invention relates to a method of bonding a piezoelectric element and an electrode, and a piezoelectric microactuator using the bonding method.
2. Description of the Related Art
In recent years, a magnetic disk drive as a kind of external storage for a computer has become increasingly smaller and thinner, and has been required to reduce its power consumption. Further, high-density and large-capacity recording has also been required in the magnetic disk drive. In general, the large-capacity recording in the magnetic disk drive can be realized by increasing a recording capacity per disk. However, in the case of increasing a recording density without changing the diameter of the disk, the track pitch is decreased to raise a technical problem such that how precisely the positioning of a head element for reading and writing information on a recording track is performed. It is therefore desired to provide a head actuator having a good positioning accuracy.
To perform high-precision head positioning in a conventional magnetic disk drive, it has generally been tried to improve the rigidity of a movable portion such as an actuator arm, thereby improving a primary resonant frequency in the horizontal direction of the movable portion. However, there is a limit to the improvement in the resonant frequency. Even if the resonant frequency in the horizontal direction of the movable portion can be greatly improved, there occur vibrations due to the spring characteristic of a bearing for supporting the movable portion, thus causing a reduction in head positioning accuracy.
As one of means for solving these problems, a so-called double actuator has been proposed. The double actuator is configured by mounting a second actuator for following a track, i.e., a tracking actuator on a front end portion of the arm of the head actuator. The tracking actuator is operated independently of the head actuator to finely move a head provided at the front end of the tracking actuator, thereby attaining the tracking of the head.
As the tracking actuator in the double actuator, an actuator using a laminated type piezoelectric element has been proposed to attain precise positioning of the head. For example, two laminated type piezoelectric elements are located on the opposite sides of an actuator arm, and a voltage is applied to the piezoelectric elements in such a direction that one of the piezoelectric elements is expanded and the other piezoelectric element is contracted. As a result, the head is rotated toward the side where the piezoelectric element contracted is located.
In the conventional two-stage actuator using the laminated type piezoelectric elements, however, there is a possibility that a voltage may be applied in a direction opposite to the polarization direction of each piezoelectric element, that the piezoelectric elements may be exposed to a high-temperature atmosphere, or that the piezoelectric elements may be changed with time, for example. As a result, the piezoelectric elements are depolarized to cause a gradual decrease in displacement per unit voltage. Accordingly, a desired stroke cannot be obtained after long-time use. Further, the conventional two-stage actuator using the laminated type piezoelectric elements has another drawback such that the manufacturability of the piezoelectric elements is low and the accuracy of the outer dimensions of each piezoelectric element is necessary to cause a cost increase.
Another two-stage actuator using a shear type piezoelectric element in place of the above-mentioned laminated type piezoelectric element having many drawbacks has been proposed in Japanese Patent Laid-open Nos. 10-293979 and 11-31368. Japanese Patent Laid-open No. 11-31368 discloses a microscopic moving mechanism for a head. This mechanism has a three-layer structure such that an electrode is formed on the front end of a head arm, that two shear type piezoelectric elements having different polarization directions are mounted on the electrode, and that a movable member is mounted on the piezoelectric elements. A head suspension is mounted on the movable member.
Accordingly, as compared with the thickness of only a spacer interposed between a head arm and a suspension as a conventional structure, the thickness of the above three-layer structure between the head arm and the suspension is larger, so that this mechanism is not suitable for a reduction in thickness of the head actuator. Further, such an increase in thickness between the head arm and the suspension causes an increase in spacing between opposed disk surfaces. Accordingly, the number of disks that can be mounted in a disk drive is decreased, and as a result the storage capacity becomes smaller than that in a disk drive having the same height.
The present applicant has proposed an improved microscopic moving mechanism for a head which can solve the above problem. In this mechanism proposed by the present applicant, an actuator base bent like a crank is fixed to a front end portion of an actuator arm. A multilayer structure composed of a base electrode, shear type piezoelectric elements, a movable electrode, and a movable plate is fixed to the actuator base, and a suspension is fixed to the movable plate. Since the actuator base is bent like a crank, the upper surface of the actuator base and the upper surface of the movable plate can be made flush with each other, thereby reducing the thickness of a microscopic moving mechanism for a head using shear type piezoelectric elements.
In the above mechanism proposed by the present applicant, the piezoelectric elements and the base electrode must be electrically connected, and the piezoelectric elements and the movable electrode must be electrically connected. To this end, a conductive adhesive is used to fix the piezoelectric elements to the base electrode and the movable electrode. On the other hand, the actuator base and the base electrode must be insulated from each other, and the movable electrode and the movable plate must be insulated from each other. To this end, a usual insulating adhesive is used to fix the actuator base and the base electrode and to fix the movable electrode and the movable plate.
The use of the conductive adhesive is intended to ensure reliable bonding between the piezoelectric elements and the base electrode and between the piezoelectric elements and the movable electrode. However, since each piezoelectric element has a thickness of 0.15 mm, there is a possibility that a short circuit may be generated between the base electrode and the movable electrode because of working error or the like. Further, a curing period of about 3 minutes at 150xc2x0 C. is required for the conductive adhesive, thus greatly reducing the productivity. In addition, distortion remains in curing the conductive adhesive because it is formed of resin.
It is therefore an object of the present invention to provide a method of bonding a piezoelectric element and an electrode which can reliably bond the piezoelectric element and the electrode without the occurrence of a short circuit or the like.
It is another object of the present invention to provide a piezoelectric microactuator which can easily and reliably attain the bonding between a piezoelectric element and an electrode.
In accordance with an aspect of the present invention, there is provided a method of bonding a piezoelectric element and an electrode, comprising the steps of forming a first coating of a material selected from the group consisting of Au, Al, Zn, Cu, and Sn on a bonding surface of said piezoelectric element; forming a second coating of a material selected from the group consisting of Au, Al, Zn, Cu, and Sn on a bonding surface of said electrode; and bringing said first and second coatings into close contact with each other and heating them under pressure to form a metallic bond or intermetallic compound between said first and second coatings. Preferably, the combination of said materials of said first and second coatings is Au/Au, Au/Al, Zn/Cu, or Sn/Cu.
Preferably, each of said first and second coatings has a thickness of 1 xcexcm or more. Preferably, the method further comprises the step of ultrasonically vibrating at least one of said piezoelectric element and said electrode under the condition where said piezoelectric element and said electrode are kept in close contact with each other, before said step of heating them under pressure. Alternatively, the method may further comprise the step of irradiating at least one of said first and second coatings with a plasma selected from the group consisting of O2, Ar, N2, SF6, and CF4, before said step of heating them under pressure.
In accordance with another aspect of the present invention, there is provided a piezoelectric microactuator comprising an actuator base; a base electrode fixed to said actuator base; first and second shear type piezoelectric elements bonded to said base electrode by a first metallic bond of Au/Au or by a first intermetallic compound selected from the group consisting of Au/Al, Zn/Cu, and Sn/Cu, said first and second piezoelectric elements having polarization directions perpendicular to the direction of their thicknesses and opposite to each other; a movable electrode bonded to said first and second piezoelectric elements by a second metallic bond of Au/Au or by a second intermetallic compound selected from the group consisting of Au/Al, Zn/Cu, and Sn/Cu; and a movable plate fixed to said movable electrode.
In accordance with a further aspect of the present invention, there is provided a piezoelectric microactuator comprising an actuator base; a base electrode fixed to said actuator base; first and second shear type piezoelectric elements bonded to said base electrode by a metallic bond of Au/Au or by an intermetallic compound selected from the group consisting of Au/Al, Zn/Cu, and Sn/Cu, said first and second piezoelectric elements having polarization directions perpendicular to the direction of their thicknesses and opposite to each other; a movable plate fixed to said first and second piezoelectric elements; a first wire for connecting said first and second piezoelectric elements; and a second wire for connecting one of said first and second piezoelectric elements to said base electrode; said base electrode having a first conductor pattern electrically connected through said metallic bond or said intermetallic compound to said first and second piezoelectric elements, and a second conductor pattern electrically independent of said first conductor pattern and connected to said second wire.
In accordance with a still further aspect of the present invention, there is provided a method of bonding a piezoelectric element and an electrode, comprising the steps of preparing a Snxe2x80x94Pb solder containing 0.1 to 10 wt. % of a material selected from the group consisting of Ag, Bi, Sb, Ge, and Ni; and soldering said piezoelectric element and said electrode by using said Snxe2x80x94Pb solder.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.